[wonton53]

> Practically though, it doesn't happen very often

It probably happens much more often than you think if you say this as someone with an insider perspective. I have had to integrate with banking systems and payment systems that use this approach, and it is extremely frustrating and comes of as a way of offloading work to the client. If a payment capture succeedes but subsequently always returns 500 an api client has to first query the status and then execute if-then logic for something that could easily just have been a retry of the request (3x the logic). This is acceptable since there are a million other ways to mess up idem potency so integrations kind of end up this way anyway. But the worst part of such an approach is debugging the issues as a client. A client cannot see the internal logs and therefore have to call support which will ALWAYS answer «the transaction seem fine with us» and basically just close the case to protect their KPIs. Im pretty sure this type of issue has a name but cannot find it (state client see dont match the real state). I dont mean to offend the work people put into these APIs, but I cannot see the good qualities of this approach other than saving development hours (and possibly saving one db query, but as an effect you get a status request plus another capture request as a workaround from your clients).

[brandur]

Just to clarify: I was speaking specifically about the case where you have a series of DB calls (like: auth user, retrieve account record, insert idempotency key, do more stuff), and the first one succeeds and the next ones fail. It can happen where the DB suddenly drops out as the request is executing, but it's more likely that it's either available or it isn't, so either the request succeeds, or it can't start.

And this comment was just meant to talk about faults with your own database. Once you are reaching out to other systems you see all kinds of problems regularly, but those tend to be handled more robustly because you kind of have to.

[wonton53]

I might have misunderstood the scope of the idempotency layer, I thought it was this thin layer close to the web server that just replays the last answer from the services that are lower down. So that means stripe actually goes down to some store or external system to check the transaction status for subsequent calls with the same key?

I know how complicated integrating with stuff like the bank, 3Dsecure, AML and fraud prevention is. One of the systems I integrated with had no refund functionality, they expected a manually initiated bank transfer from our customer bank to the end user! So I certainly understand some states are irrecoverable in a web request. It is important to do though, because it saves all clients work for each automated recovery that can be done.